1. Field of the Invention
The present invention relates to a paste for a solar cell, and more particularly, to an electrically conductive paste used to form electrodes of a back contact-type solar cell.
2. Technical Background
Silver paste is widely used for the electrode paste used in solar cells, since electrode pastes for solar cells are required to have low electrical resistance to facilitate improved efficiency. In the case of contact between a metal and semiconductor, a Schottky barrier is known to be formed that causes a considerable increase in contact resistance. This type of contact is referred to as Schottky contact. Since the electrical resistance of an electrode is the sum of the conductor resistance and contact resistance, in addition to lowering the conductor resistance of an electrode paste, it is also necessary to reduce the contact resistance with the conductor. The ideal contact of an electrode material, free of a Schottky barrier, is referred to as Ohmic contact; and contact resistance is known to decrease in this contact state.
Methods for realizing Ohmic contact at the interface between a metal and semiconductor consist of either reducing the Schottky barrier height from the semiconductor to zero, or reducing the thickness of the electrode material to narrow the width of the Schottky barrier and approach the behavior of Ohmic resistance due to a tunnel effect.
Examples of such methods include a technique in which the thickness of a layer formed between a metal and semiconductor for electrical continuity is decreased, a technique in which multilayer thin films which have similar work function are inserted between the metal and semiconductor, a technique in which a film interposed between the metal and semiconductor is subjected to high-temperature heat treatment to form an alloy, and a technique in which contact resistance is decreased by high-temperature annealing. In the case of solar cells, however, high-temperature treatment results in the risk of lowering the power generation efficiency of the cell. In addition, in the case of the application of thin films technique, the production process is time-consuming and complex, thereby preventing production from being carried out at low cost.
Recently however, studies have been conducted on back contact-type solar cells for the purpose of further enhancing the power generation efficiency of solar cells. Back contact-type solar cells refer to solar cells in which the electrodes are formed on the opposite side from the sunlight receiving side (back side), thereby making it possible to increase the light receiving surface since the electrodes are not formed on the light receiving surface. The structure of conventional crystalline solar cells in which the electrodes are formed on the light receiving surface typically consisted of the formation of an n layer on the light receiving side of a p-type semiconductor. Consequently, electrically conductive paste for electrodes used on the light receiving side was also developed for use with an n-type semiconductor. An aluminum paste is used to utilize BSF effects on the back side. Since aluminum has an inferior adhesive strength as compared with solder, silver paste is used at those portions requiring soldering. This silver paste for the back side is used primarily for the purpose of ensuring electrical connection with the aluminum paste and physical connection with the substrate, and Ohmic resistance with the p-type semiconductor is not required.
However, in the case of back contact-type solar cells, it is necessary to develop electrodes having low contact resistance with the p-type semiconductor layer and to develop an electrically conductive paste for back contact applications.
Moreover, the method for forming electrodes employs a thin film process using vapor deposition or sputtering, and a thick film process using a paste. It is preferable to form the electrodes using a thick film process in consideration of production costs. In order to minimize the decrease in solar cell efficiency caused by thermal damage, an electrode formation method is required that allows electrodes to be formed at a low temperature and in a short period of time. Low-temperature, short-duration production is also important with respect to costs and production efficiency.
In the case of solar cells in which electrodes are formed on both sides, the light receiving side paste usually contains as basic components electrically conductive particles in the form of Ag, binder, glass frit and a solvent (see, for example, Japanese Patent Application Laid-open No. 2006-295197). In Japanese Patent Application Laid-open No. 2006-295197, examples of electrically conductive particles include metal particles selected from the group consisting of Cu, Au, Ag, Pd, Pt, alloys of Cu, Au, Ag, Pd and Pt, and mixtures thereof. On the other hand, a paste composed primarily of aluminum is typically used for the paste for forming sites on the back side that contact the semiconductor.
The present invention provides a paste able to be applied to a back contact-type solar cell having low contact resistance between formed electrodes and a semiconductor.
In addition, the present invention provided a method for forming electrodes having low contact resistance with a semiconductor on the back of a solar cell substrate using an electrically conductive paste.